1. Field of Invention
The present invention relates to a projection-type display apparatus with a color light synthesizing device and an illumination optical system for the apparatus.
2. Description of Related Art
Projection-type display apparatuses for projecting a color image onto a projection screen employ a cross-dichroic prism in many cases. In a transmission-type liquid-crystal projector, a cross-dichroic prism is used as color synthesizing means for synthesizing three color light beams of red, green and blue and for outputting the synthesized light beam in the same direction. In a reflection-type liquid-crystal projector, a cross-dichroic prism is used as color separation means for separating a white light beam into three light beams of red, green and blue while being used as color synthesizing means at the same time for synthesizing back modulated three color light beams and outputting the synthesized light beam in the same direction. One of known projection-type display apparatuses using a cross-dichroic prism is disclosed in Japanese Unexamined Patent Publication 1-302385, for example.
FIG. 16 shows a concept of the major portion of a projection-type display apparatus. The projection-type display apparatus includes three liquid-crystal light valves 42, 44 and 46, a cross-dichroic prism 48 and a projection lens system 50. The cross-dichroic prism 48 synthesizes three color light beams of red, green and blue respectively modulated by the three light valves 42, 44 and 46 and outputs the synthesized light beam to the projection lens system 50. The projection lens system 50 projects the synthesized light beam onto a projection screen 52.
FIG. 17 is a partly exploded perspective view showing the cross-dichroic prism 48. The cross-dichroic prism 48 is manufactured by gluing four right-angle prisms with right-angle surfaces interfaced by an optical cement.
FIGS. 18(A) and 18(B) are explanatory views illustrating the problem that arises when the cross-dichroic prism 48 is used. As shown in FIG. 18(A), the cross-dichroic prism 48 has a red-color light reflecting film 60R and a blue-color light reflecting film 60B, both of which are arranged in the shape of the letter X at the interfaces formed in the right-angle surfaces of the four right-angle prisms. Since an optical cement layer 62 is formed in the gaps between four right-angle prisms, both the reflecting films 60R and 60B have a gap at the central axis 48a of the cross-dichroic prism 48.
When a light beam passing through the central axis 48a of the cross-dichroic prism 48 is projected onto the projection screen 52, a dark line resulting from the central axis 48a is displayed in a presented image. FIG. 18(B) shows one example of such a dark line DL. The dark line DL, of a color different from the rest of the image, presents a slightly darker band at the center of the projected image. It is considered that the dark line DL is attributed to light scattering taking place in the gap in the central axis 48a and failure in the reflection of red-color light and blue-color light. This problem is also encountered in a cross-dichroic mirror in which two types of dichroic mirrors having selectively reflecting films, such as a red-color light reflecting film and a blue-color light reflecting film, are crossed in the letter X configuration. In this case, a dark line attributed to a central axis of the mirrors is also formed in a resulting image.
As described above, the dark line is created at the center of the screen by the central axis of the cross-dichroic prism 48 or the cross-dichroic mirror in the conventional projection-type display apparatus.